Please help me brainstorm! Multi WP Installs w/ Forum (2 posts)

Hello dear WordPress friends, I need your help to brainstorm a solution for my current project.

The site is a blog, with a section for product reviews and a user forum. The ideal setup would have the blog and reviews section completely separate with a forum in the mix, but all sharing the same login.

So far I've though of having two separate WP installs (one for the blog and one for the reviews) since both sections will have a slightly different appearance and function.

It's not entirely necessarily to have users login to the blog/review sections, I guess. So I could still have them running independently and just have users login to post on the forums.

The forums I've narrowed down to are Simple:Press or PHPbb. The advantages to Simple:Press are that users can have a shared login to the blog side of the side - where I wouldn't want to even try the PHPbb bridge. I just don't know how stable/powerful Simple:Press is. It doesn't seem to be used much, not even discussed much.

Please help me figure this out. Is it really necessary for users to login to the blog/reviews? I would really like to use the Facebook Connect App but it doesn't look stable enough yet. Any forum solutons? If I don't have users login to the blog would it be ok to just use PHPbb?